This invention relates to an acidic fluid that becomes a diverting fluid as it is pumped into a subterranean formation. More particularly, it relates to a fluid system—that contains a concentration of a surfactant-containing system that is initially unable to cause the fluid to be a diverting fluid system—that becomes a diverting fluid system as the fluid flows through a permeable subterranean formation. Most particularly it relates to a method of treating a subterranean formation with such a fluid system.
In many oilfield operations there are occasions when it is desirable to pump a fluid that initially flows easily through a formation but then flows with much more difficulty after it has been injected into a borehole (a wellbore) and has progressed a certain distance into the formation. Initially, the ease of flow might be desirable, for example, to reduce the hydraulic horsepower needed for pumping, to facilitate mixing procedures at the surface, or to decrease the resistance to flow of the fluid through the pores of a subterranean formation. Subsequently, more difficult flow might be desired, for example to reduce the flow rate of the fluid through a porous formation or through fractured or lost-circulation zones, for example so that the fluid acts as a diverter. Currently, there are several types of acid systems that are designed to increase in viscosity, but the viscosity increases are dependent upon consumption of some or all of the acid. Some of these systems are polymer-based; polymers may damage formations. Other systems are based on viscoelastic surfactants (VES's) that are much cleaner; however, most VES systems require that the initial fluid contain sufficient surfactant to form a gel, and surfactants may be expensive.
We recently found that certain VES fluid systems can form diverting fluids when injected at concentrations below the concentrations at which they form viscoelastic fluids. It would be desirable to find additional low-viscosity fluids that would increase their resistance to flow after being injected into porous materials.